Data compression is used in communications and computer networking to store, transmit, and reproduce information efficiently. It finds particular application in the encoding of images, audio and video. Common image compression formats include JPEG, TIFF, and PNG. A newly-developed video coding standard is the ITU-T H.265/HEVC standard. Other video coding formats include the VP8 and VP9 formats developed by Google Inc. Evolutions to all of these standards and formats are under active development.
All of these image and video coding standards and formats are based on predictive coding that create a prediction of data to be coded, then encode the error (often called the residual) in the prediction for transmission to a decoder as a bitstream. The decoder then makes the same prediction and adjusts it by the reconstructed error decoded from the bitstream. The data compression of the error at the encoder often includes a spectral transform of the error to create blocks of transform domain coefficients. This is typically accompanied by lossy quantization. The reverse operations are performed at the decoder to reconstruct the error/residual. Entropy coding (often context-adaptive) is typically used to encode the residual data, plus side information for making the predictions (e.g. intra-coding mode or inter-coding motion vectors), to generate a bitstream for transmission from encoder to decoder or for storage on a storage medium for later decoding. In some cases, the error at the encoder is encoded without using a spectral transform and/or quantization.
In the case of intra-coding, i.e. spatial compensation, the prediction is based on previously-reconstructed nearby pixel data from within the same picture, typically pixels in a neighbouring block. Traditional intra prediction methods are mostly based on the concept of linear displacement (directional prediction). The encoder losslessly encodes the intra-coding mode (i.e. the selected direction from among the plurality of possible directions) for building a prediction block, and the decoder decodes that intra-coding mode information in order to be able to build the same prediction block in the same way.
One of the principal objectives of any proposed change to video coding is to improve the balance between compression and perceptual quality. Because intra-coding modes are to be losslessly encoded, changes to intra-coding mode encoding and decoding focus on improved compression. Any savings in terms of bits for communicating intra-coding modes from encoder to decoder will come from a reduction in the bitrate needed to encode intra-coding modes. It may be advantageous to find a more efficient way to encode intra-coding mode selection.
Similar reference numerals may have been used in different figures to denote similar components.